1. Field of the Invention
The invention relates to an apparatus, preferably for a motor vehicle, in particular for a commercial vehicle, for example a lorry or an omnibus, that makes selective and therefore combined electrical utilization and mechanical utilization of the energy of an expansion machine of, for example, a mobile waste heat utilization system possible and to a corresponding operating method.
2. Description of the Related Art
In systems for waste heat utilization (Waste Heat Recovery=WHR) of internal combustion engines, steam circuits, inter alia, are used. Here, a circuit medium has to be brought to a high pressure level, evaporated in an evaporator, and superheated. The steam is fed to an expansion machine that converts the energy stored in the steam into utilizable energy. Downstream of the expansion machine, the steam is liquefied again and is fed to a feed pump.
It is known from practice to utilize the drive energy generated by the expansion machine in two ways, namely either electrically or mechanically.
Electrical Utilization:
Here, the expansion machine is coupled to a generator. The generator converts the mechanical energy of the expansion machine into electrical energy. The generated electrical energy is fed to the internal combustion engine again via an electric motor and therefore reduces the fuel consumption of the internal combustion engine.
The use of the electrical energy generated to maintain the on-board power network of a motor vehicle is therefore not expedient, since the provided performance (for example, 5 kW in a main operating range) lies above the required performance (for example, 2 kW). Rather, a plurality of previously mechanically driven components would have to be electrified, in order to increase the requirement for electrical energy in the motor vehicle (for example, power steering pump, coolant pump, various actuators). This results in the problem that the energy from the WHR system is not permanently available to its full extent. For example, the system requires some minutes to heat up after cold starting, until the expansion machine can be operated. At high temperatures, the energy that can be generated of the expansion machine of the WHR system is also reduced on account of limited performance of the vehicle cooling system. In these cases, the high electrical energy requirement of the motor vehicle would then have to be generated in a different way, which can require additional generators on the internal combustion engine.
However, electrical utilization of the expander energy permits an attachment of the expansion machine independent of the engine with regard to rotational speed and positioning. This leads to it not being absolutely necessary for the expansion machine to be installed on the internal combustion engine and to it being possible for the expansion machine to be operated in an optimum rotational speed range despite fluctuating internal combustion engine rotational speeds. This advantage can compensate for or even overcompensate for the disadvantage of the expander-generator-electric motor-internal combustion engine efficiency chain. A further advantage of the electrical utilization consists in it being possible for the electrical energy to be buffer-stored in combination with a battery. As a result, the WHR system can also generate energy when the internal combustion engine is, for example, in braking mode. The buffer-stored energy is then made available to the internal combustion engine again via the electric motor in a delayed manner.
Generators for electrical utilization of the expander energy will usually operate with operating voltages of greater than 24 V on account of the required power outputs. In principle, fast-rotating turbomachines (steam turbines) are more suitable for coupling to a generator than working chamber-forming machines (for example, piston engines) at lower rotational speeds, since fast-rotating generators are of more compact design.
Mechanical Utilization:
Here, the expansion machine is coupled mechanically to the internal combustion engine via a transmission. It is an object of the transmission to convert the rotational speed and to bridge spacings between the expansion machine and the internal combustion engine, and to connect and disconnect the expansion machine to/from the internal combustion engine. The transmission can consist of a drive in various embodiments (for example, friction wheel mechanism, spur gear mechanism, planetary gear mechanism, chain drive, and/or belt drive, etc.) or combinations thereof and an active or passive switching element for connecting and disconnecting the expansion machine and internal combustion engine (for example, clamping body freewheel, switchable freewheel, electrically or hydraulically actuated multiple disc clutch, etc.). The transmission feeds the mechanical energy of the expansion machine to the internal combustion engine again and therefore reduces the fuel consumption of the internal combustion engine.
One advantage of the mechanical coupling is that the expander energy does not always have to be permanently available. If the WHR system is active, the additional mechanical energy of the expansion machine leads to a reduction in the fuel consumption; if the system is not ready for operation, the internal combustion engine compensates for this missing energy.
However, a mechanical utilization concept of the expander energy results in an attachment of the expansion machine that is dependent on the engine with regard to rotational speed and positioning. This leads to it being absolutely necessary for the expansion machine to be installed on the internal combustion engine, in order for it to be possible to introduce the energy into the crankshaft of the internal combustion engine by way of paths which are as short as possible. In addition, a fluctuating engine rotational speed also leads to a fluctuating expander rotational speed, as a result of which the expansion machine cannot always be operated in an optimum rotational speed range. This disadvantage can be compensated for or even overcompensated for by way of the advantage of the short expander-transmission-internal combustion engine efficiency chain, since modern transmissions have very high degrees of efficiency. The disadvantage of the rotational speed fluctuations can also be reduced by way of the selection of those expander types which have a very flat characteristic curve in the efficiency/rotational speed characteristic diagram.
A further disadvantage of the mechanical utilization consists in that no energy can be buffer-stored. As a result, the WHR system cannot generate any energy when the internal combustion engine is, for example, in braking mode. The energy stored in the circuit on account of the thermal inertia has to be conducted past the expansion machine and therefore cannot be used in a fruitful manner.
In principle, working chamber-forming machines (for example, piston engines) at lower rotational speeds are more suitable for mechanical coupling via a transmission than fast-rotating turbomachines (steam turbines), since the transmission can be of simpler configuration.